Current issues with acetaminophen hepatotoxicity—A clinically relevant model to test the efficacy of natural products

Abstract

There is a significant need to evaluate the therapeutic potential of natural products and other compounds purported to be hepatoprotective. Acetaminophen-induced liver injury, especially in mice, is an attractive and widely used model for this purpose because it is both clinically relevant and experimentally convenient. However, the pathophysiology of liver injury after acetaminophen overdose is complex. This review describes the multiple steps and signaling pathways involved in acetaminophen-mediated cell death. The toxicity is initiated by the formation of a reactive metabolite, which depletes glutathione and binds to cellular proteins, especially in mitochondria. The resulting mitochondrial oxidant stress and peroxynitrite formation, in part through amplification by c-jun-N-terminal kinase activation, leads to mitochondrial DNA damage and opening of the mitochondrial permeability transition pore. Endonucleases from the mitochondrial intermembrane space and lysosomes are responsible for nuclear DNA fragmentation. Despite the oxidant stress, lipid peroxidation is not a relevant mechanism of injury. The mitochondrial dysfunction and nuclear DNA damage ultimately cause oncotic necrotic cell death with release of damage-associated molecular patterns that trigger a sterile inflammatory response. Current evidence supports the hypothesis that innate immune cells do not contribute to injury but are involved in cell debris removal and regeneration. This review discusses the latest mechanistic aspects of acetaminophen hepatotoxicity and demonstrates ways to assess the mechanisms of drug action and design experiments needed to avoid pitfalls and incorrect conclusions. This review should assist investigators in the optimal use of this model to test the efficacy of natural compounds and obtain reliable mechanistic information.

Introduction

The search for new drugs and novel therapeutic intervention strategies increasingly includes testing plant extracts and other natural products. In addition, more products of traditional Eastern medicine are being evaluated. Independent of whether extracts are considered or if individual ingredients of a mixture are tested, the pharmacological efficacy of these chemicals needs to be investigated. For compounds that are assumed to have hepatoprotective effects, the model of acetaminophen (APAP) overdose in rodents, especially mice, is one of the most popular experimental in vivo systems used today (Campos et al., 1989, Chen et al., 2009, Gao and Zhou, 2005, Hau et al., 2009, Hsu et al., 2008, Küpeli et al., 2006, Wang et al., 2010, Wu et al., 2008, Wu et al., 2010, Yuan et al., 2010). The advantage of this model is that APAP is a dose-dependent hepatotoxicant, the experiments are technically easy to perform and, most importantly, it is a clinically relevant model. However, after more than 35 years of research there is substantial information in the literature on mechanisms of APAP hepatotoxicity (Hinson et al., 2004, Jaeschke et al., 2003, Jaeschke and Bajt, 2006, Jaeschke and Bajt, 2010, Nelson, 1990, Nelson and Bruschi, 2001). Some of these mechanisms are well established and others are more or less controversial; some are correct and some are skewed by experimental conditions making them not clinically applicable. In addition, many aspects of APAP-induced cell death and liver injury are still unknown. Thus, the mechanisms of APAP hepatotoxicity are extremely complex and the interpretation of in vivo data is difficult. Unfortunately, the model is being used as a tool by investigators who are not necessarily experts in APAP toxicity leading to frequent misinterpretation of data (Jaeschke et al., 2010). Therefore, the purpose of this review on APAP hepatotoxicity is to discuss some of the established and controversial mechanisms and the potential pitfalls one should be aware of when using this model.